The present invention relates to a process for purifying aqueous glyoxal solutions comprising at least one acid with an ion exchanger solution and to the aqueous glyoxal solutions obtainable by this process.
Glyoxal is used, for example, as an auxiliary component in the textile or paper industry. Glyoxal is prepared typically by oxidation of acetaldehyde of by oxydehydrogenation of the corresponding glycol over a catalytic fixed bed. The catalyst used is, for example, phosphorus-doped copper. The aqueous glyoxal solutions obtained in this process have by-products which have to be removed before the further use thereof. Typical by-products are formaldehyde, glycolaldehyde, formic acid, acetic acid, and nonvolatile acids such as glyoxylic acid, glycolic acid and oxalic acid. The glyoxal prepared from glycol by oxydehydrogenation comprises generally not more than 2% by weight of acid. For the commercial use of the glyoxal, however, acid numbers of <5 mg KOH/g are required.
For removal of the abovementioned acids, in particular of the nonvolatile acids, various processes are described in the prior art. U.S. Pat. No. 3,270,062 describes a process for purifying aqueous glyoxal solutions by treating the glyoxal solution with a solid ion exchanger. This process has the disadvantage of batchwise operation. Moreover, the ion exchangers have to be regenerated frequently owing to the high acid values of the aqueous glyoxal solutions used. For this reason, considerable amounts of dilute glyoxal solutions are obtained in this process, and so the process described in U.S. Pat. No. 3,270,062 cannot be operated in an economically viable manner.
It is additionally known that acidic impurities can be removed from aqueous glyoxal solutions by treating them with a solution of high molecular weight tertiary amines or of quaternary ammonium salts in bicarbonate form in an organic solvent. In this process, the two solutions are conducted in countercurrent to one another in a multistage extraction column. This achieves continuous purification, but long residence times are required. Moreover, in this process, avoidance of excessively great glyoxal losses necessitates reextraction of the organic phase with water. The result of this is that this process is unattractive.
Workup of the glyoxal prepared by oxidation of acetaldehyde or by oxydehydrogenation is absolutely necessary since the glyoxal solutions obtained by this process have a strong yellow color. For the commercially used glyoxal solutions, typically color numbers of <200 Apha are required in the end product.
DE 34 02 733 describes a process for purifying aqueous glyoxal solutions by extracting the acids present in the glyoxal solution with a solution of a tertiary amine in an organic solvent.
In the process described therein, aqueous glyoxal solutions having an acid number of <5 mg KOH/g and a low color number are obtained. The glyoxal solutions obtainable by this process thus meet specifications required for commercial glyoxal solutions. In the process described in DE 34 02 733, the glyoxal loss into the solution of tertiary amine and organic solvent is additionally low.
The ion exchanger solution is regenerated in a manner known per se by contacting with basic compounds such as sodium hydroxide, potassium hydroxide and sodium bicarbonate, and neutralization of the acid bound in the ion exchanger solution. Typically, the neutralized ion exchanger solution still comprises considerable amounts of salts formed in the neutralization, which would be transferred to the glyoxal solution in the acid extraction and would impair the product quality. Therefore, before the regenerated ion exchanger is used again as an extractant, a salt wash with water is carried out.
A problem in the acid extraction is the glyoxal loss through glyoxal transferred to the extractant phase. It is partly degraded by Canizarro reaction in the course of regeneration of the ion exchanger solution; the degradation products pass into the wastewater.